The Facts on Mercury
What is Mercury?
Mercury (Hg) is released naturally to the environment by volcanoes, the breakdown of rocks known as weathering, and other processes (UNEP 2003). Human beings have extracted mercury for thousands of years for use in products such as paints and thermometers and processes such as gold mining. Prior to industrialization most of these uses released only small amounts of mercury into the environment. Human activities in recent decades, particularly coal combustion, are responsible for a three-fold increase in global mercury deposition (UNEP 2003). That increase reaches four- to six-fold in the northeastern United States (the Northeast) as reflected in the increase in mercury deposited to sediments of lakes (Perry et al. 2005).
Why are we concerned about mercury?
As a fundamental chemical element, mercury is persistent in the environment and does not break down or degrade. After mercury is released from ores or mineral deposits and emitted to the atmosphere, it is deposited to the Earth's surface, and some eventually flows into rivers and streams. Within watersheds and lakes, mercury is processed by specialized bacteria that convert it to methylmercury a form that is more readily absorbed in the digestive system of animals and magnified to high levels in the food web. Methylmercury binds to proteins and can bioaccumulate through the food web in fish, birds, and other wildlife. Most people and wildlife are exposed to methylmercury by eating fish, an important source of animal protein. In human populations, the most at-risk and sensitive individuals include women of childbearing age who may become pregnant, nursing mothers, and children under 12 years of age. The people with the highest exposure due to fish consumption habits include: recreational fishers and their families, some Native American populations, Asians and Pacific Islanders, and individuals who fish to meet their or their families' nutritional needs (subsistence fishers).
(Paraphrased from Mercury Matters, A Science Links Publication of the Hubbard Brook Research Foundation)
Is it Safe to Eat Fish?
People are exposed to mercury mainly through eating fish and shellfish -- and 95 percent or more of the mercury in fish is the more toxic methylmercury. According to EPA, fish fillets containing more than .3 parts per million of methylmercury should not be eaten (Canada and the states of Maine and Minnesota suggest you avoid fish with .2 ppm). Fish caught in water with very low concentrations of mercury (less than 1 part per trillion) can nonetheless contain toxic levels of methylmercury. In some aquatic ecosystems, the concentration of methylmercury increases 10 million times as it makes its way up through the food web from microscopic algae to shark and tuna.
Mercury concentrations in fish from lakes and rivers throughout the United States now exceed the mercury levels that are cause for human and wildlife health concern. As of 2008, all 50 states and 1 territory have fish consumption advisories for mercury. In addition, all states on the Atlantic and Gulf of Mexico have coastal fish advisories. (put link here to FDA fish advisory and EPA fish advisory) Although much of the scientific research on mercury in fish has focused on freshwater ecosystems, most Americans are exposed to mercury through seafood consumption.